I don't think the comma is necessary there, but there's a lot of confusion about comma placement, so maybe some people do.

I fix many kind of machine for work,but I couldn't fix it...

I fix many kind of machines for work, but I couldn't fix it...

You don't fix just one kind of machine do you? No, you fix, "many," so I made it plural.

I'm going to contact to a landlord.

I'm going to contact the landlord.

First, we don't say I'm going to contact to anyone because the second "to," is unnecessary. We say, "I'm going to contact the landlord."

Second, I changed "a," to "the," because I think you're talking about one, specific landlord. Usually when you're speaking about a specific thing (of many things of the same type), you should say, "the."